The invention relates to toothed belts adapted for synchronous motion transmission, but more particularly, the invention relates to a toothed belt constructed with materials of requisite properties that combine with each other to give the belt significantly improved performance characteristics over prior art belts.
Current methods for improving performance of toothed belts are to alter belt tooth profile, tooth pitch, tooth reinforcement material, elastomer compounding, and even sprocket configuration. Improvements in belt load carrying life have increased in small increments in response to such alterations. Several belt tooth profiles have evolved and may generally be characterized in longitudinal cross section as trapezoidal as shown in U.S. Pat. No. 2,507,852; round or curvilinear as shown in U.S. Pats. Nos. 3,756,091 or 4,037,485; and truncated round as shown in U.S. Pat. No. 3,977,265. It is estimated that over 90 percent of the toothed belts used for other than fractional horsepower transmission, are of the type with an elastomeric matrix of rubber, synthetic rubber, or blends thereof that form an outer layer and a toothed inner layer which together sandwich a spirally wound cord or cable that forms a tensile member. The outer layer optionally includes some type of embedded fibrous reinforcement whereas the inner layer forming the belt teeth customarily has a fibrous reinforcement embedded along its exterior surface to provide a wear-resistant surface while simultaneously enhancing tooth shear strength. Examples of belts with fabric along the peripheral surface of their inner layer are shown in U.S. Pats. Nos. 2,507,852; 4,037,485; 3,937,094; and 3,964,328.
U.S. Pat. No. 3,937,094 teaches that belt performance and life are improved by increasing tooth shear modulus with two layers of fabric at the tooth peripheral surface. Another technique to enhance belt peformance is to change the belt elastomer. For example, belts constructed with urethane may exhibit improved performance in terms of life and horsepower transmitting capability over similarly constructed belts made with rubber because urethane is generally recognized as being "tougher" than rubber. An example of such a belt is described in U.S. Pat. No. 3,964,328.
A need for what is described as "a substantially inextensible tensile member" is recognized for all such belts to maintain a tooth-to-tooth pitch so that a belt will satisfactorily mesh with a sprocket. However, as recognized in U.S. Pat. No. 4,047,44, a tensile member is really not inextensible and belt performance may be enhanced by altering the pitch of driver and driven sprockets to adjust for belt length changes at different power loads. Fiberglass with a greige Young's modulus of 10.0.times.10.sup.6 psi is the predominant material presently used in rubber-type toothed belts. Occasionally, metal cable with a Young's modulus of 20.times.10.sup.6 psi is used as a tensile member. Aramid fiber with a greige Young's modulus 90.times.10.sup.6 psi has been used in toothed belts made with rubber or urethane elastomers. An example of an aramid fiber having a yarn modulus of 9.0.times.10.sup.6 psi is sold under the trademark "Kevlar" by Du Pont. Aramid fiber with a yarn modulus of 9.0.times.10.sup.6 psi is herein referred to as "aramid, type I" or "aramid type I fiber."
Whatever the elastomeric material, a belt tensile member is typically formed of helically spiralled cords or cables lying side-by-side at a desired number of cords per unit belt width so that the belt has a requisite tensile modulus per unit width to control belt strain for maintaining a satisfactory tooth pitch. The load carrying lives of toothed belts tend to be unaffected by the type of cord tensile material - provided that some minimum tensile modulus is maintained. It should be noted that the process of forming a tensile member from greige cord usually decreases the tensile strength of the cord because the cord is severally bent and twisted during its processing. Nevertheless, greige cord characteristics are listed in the following examples for comparative purposes because the tensile members are formed in somewhat similar manners (i.e., they are all bent and twisted during forming).